Enabled by freedom and capacity of printed electronics with the promises of low cost and rapid manufacturing of light weight and flexible electronic circuits, printed Radio Frequency (RF) and Microwave (MW) devices such as filters, transistors, switches and antennas are emerging with different form factors. Tunability is an important functionality in various adaptive RF and MW applications such as phased array antennas, conformal antennas, and tunable frequency selective surfaces.
Ferroelectric varactors are of special interest for RF and microwave applications due to their higher performance, and reduced power consumption, size and cost. Among various ferroelectric materials, Barium Strontium Titanate (BaxSr1-xTiO3, or BST), a perovskite-type compound, is the material of choice since its Curie temperature, TC, is controllable by the Ba fraction. For x<0.7, TC is below room temperature, hence BST is in the paraelectric (nonpolar) phase at room temperature and is characterized by high dielectric constant, high tunability, low loss tangent, and high switching speed. In addition, its relative permittivity is decreased in a nonlinear fashion with no hysteresis when a bias voltage is applied. In conventional parallel-plate or coplanar-plate varactors, BST is usually used in form of thin or thick ceramic films. From the printed electronics perspective, utilizing BST in the form of ceramic films is impractical since they are brittle in nature and their fabrication require sintering at extremely high temperatures (>850° C.), which is unfeasible on plastic substrates, as well as in many semiconductor processing applications.
One promising solution to this problem may be a multiphase BST/polymer composite made by suspending micro- or nano-BST particles in a polymer matrix. Such a solution is a compromise between the processing flexibility of a polymer and the desired ferroelectric properties of BST. The fabrication and material properties of many BST/polymer composites have been reported using various polymers such as silicon-rubber, polymethylmethacrylate (PMMA), polyphenylene sulfide (PPS), and cyclic olefin copolymer (COC). However, these studies utilized plastic extrusion and injection molding. Moreover, none of these studies reported characterization of BST/polymer dielectrics at frequencies above 1 GHz.
There is a need for printed tunable elements that can be fabricated on various substrates, including flexible substrates.